It is often necessary to intravenously supply patients with pharmaceutically active liquids at a controlled rate over a long period of time. There are many applications in academic, industrial and medical fields as well as others, that benefit from devices and methods that are capable of accurately and controllably delivering fluids, including liquids that have a beneficial effect when administered in known quantities at controlled rates. This is particularly true in the medical field where treatments for many patients include the administration of a known amount of a substance at predetermined intervals. It is often desirable that this be accomplished while the patient is in an ambulatory state. A few devices have been developed in the past for accomplishing this purpose.
Prior art disposable devices typically incorporate an inflatable elastomeric balloon-like bladder forming a liquid container, with an exit flow restriction device and tubing (intravenous, or IV) for transfer of the liquid to the patient. The walls of the bladder stretch and expand when filled with the liquid, and in tension provide pressure for expelling the liquid. These prior art devices are typically filled by hand syringe and generally require excessive force to initiate the filling: it is difficult to insert an initial volume in conventional inflatable elastomeric bladders. From such balloon-bladders the pressure and flow rates typically varies widely through the course of expelling the fluid and infusing the patient, and they characteristically exhibit a rapid rise in flow rate as the balloon-device nears completion of delivery. Further, such conventional bladders have a history of many reported occasions of inflated-bladder rupture. In addition, they typically leave a significant undispensed residual volume: wasting costly pharmaceuticals.
Various materials are used for constructing conventional inflatable elastomeric bladders, including natural rubber. Construction typically requires several layers of material. The use of silicone in tubular form to function as an elastic pressurized liquid reservoir for infusion purposes is described in U.S. Pat. No. 4,909,790; this discloses an infusion device using tubular bladders mounted on mandrel supports with downstream restrictors to deliver uniform flow rates. Another example may be found in U.S. Pat. No. 7,704,230 which describes a similar pressurized fluid reservoir made from a silicone tube for an infusion system. Prior art references point to numerous possible combinations of silicones, structural dimensions, filling pressures, operating pressures, and fill volumes. However, limitations of this expandable-bladder prior art are exemplified by the performance of the silicone tube as disclosed in U.S. Pat. No. 7,704,230. The described performance is inferior because of the variability in flow rate and pressure during the infusion period, and the difficulty in dispensing substantially all of the liquid by the end of the infusion period.
Another approach to controlled infusion has been mechanical infusion pumps. In prior art, vacuum-driven or spring-driven syringe devices have been employed to create the desired fluid pressure. A typical disposable spring infuser compresses a contained bag to expel the medication. The compressed-spring mechanism squeezes the medication out of the container and through the flow-restriction device and the IV line to the patient. Since the spring pressure will decrease as the spring extends, the infusion rate will perforce decrease as the infusion proceeds.
Other prior art in infusion pumps includes electronic pumps: typically costly, non-disposable, and requiring meticulous cleaning between cycles. These are in general unsuitable for ambulatory patients.
Therefore, there is a need for a simple fluid infusion pump that is small enough to be worn by ambulatory patients, that delivers the fluid at a constant flow rate, that exhibits no burst of flow near the end of delivery, that expels essentially all of the costly fluid, and that is low enough in manufacturing cost to enable it to be single use: optionally pre-filled and disposable when the delivery is complete.